It has been a common practice to provide lift trucks with two or more hydraulic pumps for supplying the different hydraulic circuits required for the various load handling and vehicle operating functions. For example, one pump having a relatively high flow capacity is used for supplying the load handling actuator circuits, such as the lift and tilt circuits for the extendable upright. A smaller pump is provided to supply the hydraulic circuit of the vehicle power steering. It is also a known practice to provide additional pumps for other hydraulic circuits on the vehicle. In some instances, two or more pumps are connected in separate and independent hydraulic circuits and in other instances, the pumps are connected in common circuits. Typically, in a lift truck, the flow and pressure requirements of the power steering circuit are much lower than the requirements of the lift circuit. When the requirements of the various hydraulic circuits are met by using plural pumps, thepumps must be sized according to the maximum flow requirements of the respective circuits. Each additional pump and its attendant drive arrrangement, hydraulic and electrical circuit components requires additional space and adds considerable weight to the vehicle. Accordingly, it is desirable to eliminate the need for plural hydraulic pumps and the duplication of circuit components in the hydraulic system of a work vehicle.
It has not been practical to use a single pump such as the large pump required for the lift circuit, which may have a by-pass pressure of 3000 PSI, for supplying the power steering circuit which needs only five gallons per minute at 1000 PSI. Certain load handling circuits such as a tilt circuit for the upright and an auxiliary circuit for a side shifter may be operated from the large pump of the lift circuit because each use is of short duration. However, other load handling circuits such as auxiliary circuits for clamps and rotators are not desirably supplied from the pump for the lift circuit because such devices are used frequently and for prolonged periods.
In the prior art, a lift truck is known in which a single pump supplies hydraulic fluid for both the lift circuit and for the power steering circuit. In this system, the outlet of the pump is supplied through a priority valve or flow divider to both the lift circuit and the power steering circuit which are both returned to the sump. The priority valve functions to allocate, on a priority basis, a rated value of flow to the power steering circuit, the rated value corresponding to the maximum flow required for operation of the power steering circuit. The pump flow which exceeds the rated value for power steering is supplied by the priority valve to the lift circuit. The power steering circuit is provided with relatively low pressure relief and the lift circuit is provided with a separate, and relatively high, pressure relief. In this system, the flow allocated to the power steering system is wasted except to the extent that it is used in power steering since the entire rated maximum flow for power steering is returned to sump whether it is used or not. This requires a pump which is sized to deliver the maximum rated flow of the lift circuit plus the maximum rated flow of the power steering circuit. Consequently, there is a considerable amount of unused capacity and wasted power since the power steering circuit is continuously supplied with its rated flow but power steering is used only part time and the full rated flow is very seldom required.
In my prior U.S. Pat. No. 3,568,868 a single pump supplies fluid to both the tilt circuit and the lift circuit of the extendable upright. For this purpose, a flow priority valve is connected between the outlet of the pump and the inlet ports of the tilt and lift control valves so that the flow requirement of the tilt actuators is always supplied. A one way valve connects the tilt circuit to the lift circuit so that the entire flow from the pump can be supplied to the lift circuit. In this arrangement, a 48 volt battery may be substituted for a 32 volt battery without adverse consequence even with increased pump output because the flow priority valve regulates the flow to the tilt actuators and the excess flow is directed to the lift circuit.
In the Williams et al U.S. Pat. No. 3,964,260, a single hydraulic pump is utilized to supply fluid to the power steering circuit and also to the lift and tilt circuits of the extendable upright. The pump has sufficient capacity to meet the maximum flow requirements and any excess fluid is diverted to an accumulator. The steering circuit is connected to the accumulator so that sufficent flow will be available in the event of need for emergency steering.
A general object of this invention is to provide an improved hydraulic system for energizing the power steering and load handling hydraulic circuits of a work vehicle.